German Pat. No. 1630298 discloses a transmission of the general kind to which the present invention relates. The infinitely variable V-belt transmission, which is drivingly connected to the output of the hydrodynamic transmission unit, includes as is conventional a primary pulley and a secondary pulley. Each pulley is constructed of a pair of conical discs, and at least one disc of each pair is axially movable toward and away from the other disc of that pair. A primary fluid pressure cylinder controls the spacing between the discs of the primary pulley, and a secondary fluid pressure cylinder controls the spacing between the discs of the secondary pulley. The hydrodynamic transmission unit, in the system disclosed in the German patent is a torque converter, i.e. a transmission unit which on the one end serves as take-off clutch for driving away and on the other end for increasing the engine torque at low vehicle speeds, so that the range of ratios of the transmission can be increased. The hydrodynamic transmission unit, however, may also consist of a so-called fluid coupling, by which the engine torque is not increased and which only serves as take-off clutch. A general drawback of a hydrodynamic transmission unit is the continuous presence of a speed difference between the input and the output shaft, i.e. slip, which results in a loss of power. In order to eliminate this drawback, it is known to provide the hydrodynamic transmission unit with means for locking said transmission unit in a direct drive mode. The lock-up, for example in the form of a plate clutch, effects a direct, mechanical connection between the input and the output shift of the hydrodynamic transmission unit.
A drawback of the transmission lock-up feature is the complexity of its control, i.e. the regulation of the locking of the clutch in a direct drive mode during the periods in which the hydrodynamic transmission unit is not used for take off or for torque increase. For example, when the clutch is hydraulically operated a special operating valve is required.